1. Field of the Invention
This invention relates to a superconductive voltage stabilizer. In particular, the invention relates to a voltage stabilizer which utilizes the energy stored in a superconducting energy storage coil. In operation, the invention draws energy from a power line, stores that energy in a superconducting energy storage coil, and then processes that energy into a form which can be used for feeding to a load. In this way, the load is isolated from the power distribution lines, and consequently does not induce voltage or current disturbances typically induced when a load draws energy from the power distribution system.
2. Background of the Prior Art
The quality of power delivered by utility systems determines how well electrical and electronic equipment operates. Any disturbances to the power system can severely affect the equipment's performance. Power disturbances typically result from lightning, utility switching and utility outages. Such disturbances can also be created by the users of power through the switching of loads, ground faults, or abnormally high demand from heavy normal equipment operation. In each of these situations, the depletion of power through the line is severe enough to affect the operation of electrical equipment being used by other utility customers. In one example, the fluctuating load of a large welder in a mill producing wire mesh can cause lights and television sets to flicker for approximately 500 residential customers who received their power from the same feeder line used to supply power to the mill. Proposed solutions in this case included powering the equipment by a diesel generator during evening hours or installing a special electric utility line connected directly to the mill at a substantial cost.
The preceding case illustrates that a solution to power line disturbances is to upgrade the utility lines to the source of the excessive load. Such upgrading, however, is an expensive solution. Consequently, various other solutions have been proposed and are currently in use.
Many different types of power conditioning systems have been devised to prevent electrical and electronic equipment from creating or being affected by power line disturbances. Computer systems are particularly sensitive to variations which occur in the power being delivered to such systems. One solution currently being used to protect computer systems is the Uninterruptible Power Supply or UPS. The UPS isolates the computer from the power distribution line so that any changes in delivered power do not affect the computer's operation. The system is designed to automatically provide power without delay or transients during any period when the normal power supply is incapable of performing acceptably. However, the amount of current that can be provided by a UPS is limited. Consequently, such a system is unsuited for use in utility and industrial applications, particularly where motors must be started. For example, the in-rush of current necessary to start a motor is too large to be supplied by a typical UPS.
In the case of motors, electronic motor starters have been successfully employed to start motors. An electronic motor starter reduces the voltage delivered to a motor at start-up thereby decreasing the load seen by the utility system. Although this reduces power line disturbances, it also reduces the current delivered to the motor. In the case where the driven system is a large load, oftentimes the motor will not start because of the reduced input voltage. Motor starters, however, are successfully used to supply smaller currents to motors driving lighter loads.
One recent development in voltage stabilization devices is the Static-VAR Compensator. The Static-VAR Compensator uses a configuration of inductors, capacitors, and high power electronics. These devices are designed to deliver large amounts of reactive power to equipment such as arc furnaces or arc welders. However, Static-VAR Compensators cannot be used to deliver real power during voltage sags or momentary outages.
Another technique to control power disturbances is to store energy when demand is low and return that energy to the power system when demand is high. Battery systems have been used to store energy for this purpose, but battery systems have gained limited use because of various deficiencies. Efficient batteries are quite expensive, and since the amount of energy stored depends on the number of batteries used, large capacity battery systems are prohibitively expensive. Also, most batteries produce hydrogen during operation, and because hydrogen is highly flammable, battery systems can pose serious safety problems.
More recent energy storage techniques employ superconducting technology to store energy to be returned later to the utility system. Peterson U.S. Pat. No. 4,122,512 describes a system storing alternating current power in a superconductive magnet or inductor. Three-phase alternating current is converted to direct current and stored in a superconducting inductor. During periods of high energy demands, the direct current is reconverted to alternating current and delivered back to the three-phase line. In this way, any sudden depletion in line voltage is compensated by the energy stored in the superconducting system.
Higashino, U.S. Pat. No. 4,695,932 discloses an energy storage circuit which converts three-phase alternating current to direct current. The DC current is then stored in a superconductive energy storage coil. A DC capacitor and chopper circuit are used to control the amount of direct current stored in the superconductive energy storage coil. This configuration allows the current capacity of the AC supply line equipment and the thyristor converter to be scaled down in accordance with service power established by the current rating of the coil, and also allows a reduction of operation losses.
Prior art systems have reduced power line disturbances in two ways. One method has been to make a device specifically designed for a particular load such as the UPS for computers or the electronic motor starter for motors. Such systems, although correcting some power line disturbances, have created other problems. Specifically, they are usually load-specific and not interchangeable, and they often cannot supply sufficient current to maintain proper operation of the load.
The other solution has been to install additional power lines or to store energy delivered by power lines during non-peak hours, and return the energy back to the utility system during peak hours, as in the Peterson and Higashino systems. Installing additional power lines, however, is an expensive solution to the problem. Peterson's and Higashino's devices are directed to supplying current to support a large power system and not to correcting the problem at the source, namely the effects of protective devices on the transmission and distribution systems which occur during lightning strikes and faults for example, and the individual devices causing the problem of sudden high-load power consumption from the utility lines. Arc welders, arc furnaces, and motors, for example, each have specific current requirements due to their unique structures. Motors require large amounts of current only at start-up. Arc welders draw power intermittently during periods of welding. Since each piece of electrical machinery has its own individual power requirements, systems which attempt to maintain the power in a utility system generally do not prevent power disturbances, but simply correct overall deficiencies in the power lines when they occur.